In gas lasers, the laser-active medium is a gas which is excited to a plasma. To maintain the plasma state, energy must be constantly supplied. This is usually done by applying an electric field, which is capable of accelerating the free electrons. In principle, the field can be a direct-current field or an alternating field, whereby an obvious solution is to inject high-frequency energy. The advantages of high-frequency excitation are that no losses occur at a series resistor and that there are no voltage drops across the cathodes region. The simple pulsing capability of the generators is advantageous for applications with gas lasers. However, so-called high-frequency boundary layers occur in the plasma which are laser-inactive. Moreover, a comparatively expensive generator is needed.
Theoretically, the frequency of the excitation energy can vary from the range of radio frequencies up to beyond the microwave range. However, at high frequencies, for example in the 2.45 GHz range usually used for microwave applications, it is difficult to inject the microwave energy uniformly and effectively in plasmas, whose expansion is great enough for a laser configuration.
The German Published Patent Application 37 43 358 discloses a so-called "fast-flow" laser, which is operated at a frequency of 2.45 GH.sub.z. The gas that flows axially through the laser at a high speed is ignited with microwave energy even before it enters the laser. These types of lasers can produce a comparatively high power output in the kilowatt range; however, the laser gas must have a high flow-through speed in order to eliminate the dissipated heat of the plasma out of the laser-active volume.
From the technical literature (W. Ranz "Untersuchungen zur CO2-Laseranregung mit MikrowellenGasentladungen in nichtresonanten Strukturen" [Analyses of CO.sub.2 Laser Excitation with Microwave Gas Discharges in Non-Resonant Structures], Thesis (1988), FAU Erlangen-Burnberg), an excitation structure for a laser is known, in which a ceramic tube, which is situated inside a microwave guide and whose axis is aligned in the direction of propagation of the microwave energy, contains the laser-active gas. As a result of this design, the microwave power is attenuated by the plasma contained in the ceramic tube, so that the energy is injected into the plasma irregularly with respect to the longitudinal axis of the tube. Consequently, here as well, only so-called "running discharges" could be generated in a pulsed operation, so that at a time only a part of the gas is excited to the plasma state.
Furthermore, so-called slab lasers are known, in which the laser-active gas is situated between opposite surfaces of two wall parts, which are equally designed as electrodes for injecting the energy. In the case of the German Published Patent Application 37 29 053, a high-frequency electric alternating field is injected into such a gas slab laser. It advantageously foresees injecting the high-frequency energy at several locations to provide for uniform activation over the entire length.
Finally, the EP-A-O 275 023 discloses a type of gas slab laser, in which the excitation is supposed to take place with high-frequency energy in the radio-frequency range, whereby the energy is supplied via lines to the upper wall part of the laser configured in a receptacle. In this case, the frequency has an upward limitation, since otherwise standing waves would form and lead to corresponding irregularities of the plasma.
Given a discharge volume of a slab laser, it is desirable to increase the laser's power output by increasing the injected electric power accordingly. This is possible then at a constant plasma gas temperature, when the clearance between the plasma-limiting surfaces of the slab laser is reduced, since the diffusion cooling is improved by this means.
In previously known slab lasers, however, restrictions have been placed on reducing the electrode spacing because the laser-inactive boundary layers existing along the two surfaces of the slab electrodes restrict the thickness of the active plasma layer. A way out would be to increase the system frequency, for example up into the microwave range, since the thickness of the boundary layers decreases with increasing frequency.
EP-A-O 280 044 discloses a plasma apparatus, by which the microwave energy can be injected into a band-shaped plasma space. The microwave energy is thereby supplied laterally via a horn-type waveguide or a ridged waveguide to the plasma space. With this construction, one must accept that a standing wave with wave nodes forms in the plasma space with the result that no laser excitation takes place in the area of the nodes. Therefore, the volume of the plasma space is only partially used. This undesirable effect becomes particularly evident in the case of non-pulsed microwave excitation. None of the means described in the EP-A-O 280 044 is suited for stopping the occurrence of standing waves with irregular excitation of the laser gas. Even in the case of pulsed microwave excitation, considerable irregularities can occur, as well as the tendency to form filaments or arcs. The latter are undesirable, since as is generally known, in the case of the laser, they lead to a reduction in the radiant power due to time-related beam fluctuations and to an acceleration of the gas decomposition.
An arrangement is known from U.S. Pat. No. 4,513,424 in which microwave energy is injected parallel to a microwave guide through a coupling wall into a tube containing laser gas. Here, to be sure, no nodes develop over the length of the tube containing the laser gas, and theoretically a uniform plasma excitation can take place over the length of the tube; however, only a fraction of the microwave energy is coupled out of the hollow conductor into the plasma tube. In this arrangement, the major portion of the microwave energy is lost as the result of heat conversion.
Therefore, the object of the invention is to improve the energy injection into the laser gas in the case of a gas laser, so that by means of microwave energy, a uniformly excited plasma is produced in a flat, elongated volume. In particular, no wave nodes or antinodes should develop thereby in the plasma volume, and the microwave energy should be injected nearly completely into the plasma volume. In particular, it is the object of the invention to create such an arrangement, in which the clearance between the walls of the plasma space can be reduced to the extent that a slab clearance of about 1/10 mm or less results. In this manner, it should also be possible to use optical resonator configurations for gas lasers in the near infrared range according to the slab laser concept. It is known to use such configurations for gas lasers in the far infrared range and for semiconductor lasers.
The objective is solved according to the invention by the characteristics of patent claim 1 or patent claim 2 in their entirety. Further developments follow from the individual dependent claims, whereby these claims also indicate in particular the operating method of a laser designed according to the invention.